The subject matter of the present invention is a method for determining the lower thread supply, and a sewing machine having a lower thread supply monitoring device which includes a light transmitter and a light receiver, in which a light beam bundle from the light transmitter is directed through slits in the hook base and through peripheral openings situated in the hook body, tangential to the bobbin core and to the thread wound thereon, through the packing space between the flanges, and at least one of the light beams is received by the light receiver, and the received signal is forwarded to the machine control unit for the calculation of the bobbin packing diameter.
In sewing and embroidery using a sewing machine, it is known that two threads, the upper thread and the lower thread, are looped with one another. The upper thread, called the needle thread, is supplied from a spool situated on or next to the sewing machine; the size of this spool can essentially be freely chosen. The second thread, called the lower thread, is wound on a bobbin that is placed in the interior of the hook body, which is rotationally mounted and can be driven rotationally, of the sewing machine, and is held there so as to be freely rotatable. As a result, the maximum size of the lower thread bobbin is determined by the size of the hook. The wound-on lower thread quantity, or the lower thread supply, is always smaller by an order of magnitude in relation to the upper thread supply on the spool situated outside the machine housing, and therefore must be refilled or exchanged more often. In addition, during sewing operation the lower thread spool is not visible from the outside, because it is situated inside the hook housing, which is located in the lower arm. For this reason, the monitoring of the momentary lower thread supply and of the pulling off of the lower thread during the sewing process is difficult. Moreover, the situation is made more difficult by the fact that during the sewing of smaller articles, the operator will frequently wrap a partly already-filled lower thread bobbin with another thread, because no empty bobbin is present. This other thread is not connected with the one already present on the spool. After this outer thread has been used, a seam can no longer be produced, although for example 50% or 70% of another thread may for example still be wound on the bobbin core.
From the prior art, measurement devices are known with which it is attempted to determine the end or the remaining quantity of the lower thread on the lower thread bobbin held in rotational fashion in the hook, and to halt the sewing machine before the end of the lower thread is drawn out by the upper thread through the article being sewed, and in particular before additional stitches are sewed that are consequently not held onto the underside of the article being sewed by a lower thread.
From GB-A 2 296 721, an apparatus is known for acquiring the quantity of thread remaining on the lower thread bobbin of a sewing machine. Here, a beam of light from a light transmitter is sent in essentially tangential fashion through the annular space between the two bobbin flanges and the bobbin mandrel, to a light receiver. For this purpose, in the hook housing and in the bobbin housing holes are provided that are situated diagonally opposite one another, through which the light beams can be sent. The light beams are directed so as to pass very close to the bobbin mandrel, so that the end of the thread supply can be determined as soon as the light beams can pass through this region. If the bobbin is still filled with thread, the receiver cannot receive a light signal. A second sensor detects the movements of the hook, so that it can be clearly recognized whether the thread supply has been used up or whether the hook is merely standing still. Using such a device, the thread supply can be detected only shortly before the complete emptying of the spool.
U.S. Pat. No. 4,825,789 discloses another device for acquiring the thread end on the lower thread bobbin of a sewing machine. In this device as well, a light beam is directed from a light transmitter tangentially past the bobbin mandrel to a light receiver. As long as there is a supply of thread on the bobbin, the light beam cannot reach the light receiver. Only shortly before the end of the thread is it possible for these sensors to alert the operator of the machine by a signal that indicates the end of the thread in the lower thread bobbin.
Thus, both the known devices can recognize only the thread end, or the time before the end of the thread is reached. Breaks in the thread, or a determination of the momentary existing quantity of thread and thus the time at which the end of the thread will be reached, cannot be recognized using the known devices.